Methods for refining pig iron



United States Patent 3,423,202 METHODS FOR REFINING PIG IRON Jacques Ledune, Marchienne-au-Pout, and Paul Nilles, Embourg, Belgium, assignors to Centre National de Recherches Metallurgiques, Brussels, Belgium, and Socit Anonyme des Forges de la Providence,

ABSTRACT OF THE DISCLOSURE A method of refining pig iron comprises the steps of carrying out two refining stages, separated by a, deslagging stage. In the first refining stage an oxygen-containing gas is blown into a converter containing metal to be refined initially while the converter is stationary and subsequently while rotating the converter about its longitudinal axis. In the second refining stage oxygen-containing gas is blown into the converter while rotating it about its longitudinalaxis. Preferably, slight additional movement is applied to the converter towards the end of the second refining stage so as to bring about increased contact between slag and metal and thus stiffen the slag.

The present invention concerns improvements in methods for the pneumatic refining of pig iron carried out in a symmetrical, solid bottomed converter adapted to rotate about its longitudinal axis, and in particular to the application of such methods to phosphorus-containing pig iron in the manufacture of steels with a low content of phosphorus.

The method is well known at the present time of carrying out a pneumatic refining operation of pig iron in two phases -by means of an oxidising gas injected from above the bath into a symmetrical, solid bottomed converter adapted to rotate about its longitudinal axis. In the said method the first phase consists in keeping the converter in an upright position without any rotary motion and in blasting-in the oxidising gas onto and/or into the bath by means of a submerged lance, the necessary slagging agents being added in particulate and/or powder form. After intermediate deslagging, the second phase is started, during which the converter rotates about its longitudinal axis whilst the oxidising gas is blasted onto and/or into the bath, and the necessary slagging agents are introduced into the said bath in particulate and/ or powder form.

By means of this method it is already possible, without difficulty to obtain steels with a low content of phosphorus. When a more thorough dephosphorization is wanted, however, certain practical difiiculties are encountered. These arise particularly from the fact that with the blasting method used in a first phase it is not always possible to obtain a bath having the desired composition at the end of the said first phase.

To overcome this disadvantage, and in order to obtain with certainty steels having a very low content of phosphorus, it has already been proposed to carry out a supplementary intermediate deslagging in the course of the first phase. This manner of operating, although eifective, still has disadvantages; in the first place, time is lost through the supplementary deslagging, and furthermore there is a considerable loss in iron because of the high content of iron oxide (sometimes up to 20%) of the fresh slag.

The aim of the present invention is a method by which it is possible to overcome these disadvantages and to produce steels which have a very low content of phosphorus.

The method of the invention in which pig iron is refined in two phases separated by deslagging, by means of an oxidising gas injected from above the bath into a solid bottomed converter adapted to rotate about its longitudinal axis, has the essential feature that the first phase is begun with the converter fixed in upright position and is terminated with the converter rotating about its longitudinal axis; the second phase, after deslagging being conducted with the converter rotating about its longitudinal axis.

The period of rotation preceding the intermediate deslagging makes it possible easily and economically to produce steels with a very low phosphorus content in for instance lower than 0.015%, for low carbon steels (C, 0.15%), and lower than 0.025% for high carbon steels (C, 0.35

Moreover, the slags obtained have an iron content which does not exceed 10%, while maintaining the qualities necessary for their utilisation in agriculture.

Furthermore, during the refining period in the rotating phase preceding the deslagging, the foaming of the slag is less than if it had taken place in the upright phase, and it is accordingly possible to keep more slag in the refining vessel, which is obviously an advantage.

For obtaining high carbon steels, the intermediate deslagging takes place when the carbon content of the bath is greater by 0.020% to 0.70% than the carbon content that it is desired finally to obtain.

In order to be able to maintain an accurate carbon content in the steel, it has moreover been confirmed that it is necessary to ensure at the end of the refining a stiffening of the slag. If this precaution is not taken there is the risk of maintaining in the presence of the steel a highly oxidised slag which tends to bring about the decarburization of the steel, either in the refining vessel or in the teeming pot, as a result of the entrainment of a part of the said slag with the steel during the casting following on the refining operation. The decarburization can furthermore be accompanied by a second disadvantageous phenomenon, i.e. rephosphorization.

To carry out the stiffening of the slag at the end of the refining process it is already known to add a drying agent, such as for instance lime, and to ensure the mixing of the slag and the steel as a whole in the refining vessel, without blasting-in oxygen and without previous deslagging.

Tests have shown that in proceeding in this manner the slag acquires excessive compactness, and as a result certain disadvantages occur on the starting of the subsequent refining operation, if the final slag of the preceding operation is reused. These disadvantages consist in particular in difficulties in starting up the reactions, and in the impossibility of dissolving the slag in the presence of iron and of carbon.

These disadvantages can beovercome easily by using the method of the invention, to which is added a particular operation at the end of the refining process. When the metal has a carbon content approaching the desired content (this being always greater than 0.1% this operation consists in stiffening the slag by a relatively slight movement of the refining vessel in such a manner as to ensure a slight contact between the slag and the metal, the stififening of the said slag being a result of the reaction of the carbon contained in the metallic bath with the iron oxides of the slag. The decrease of iron oxides contained in the slag contributes to the partial solidification of the said slag, rendering it inert in relation to the metallic bath, and accordingly making it possible to avoid any risk of subsequent rephosphorization.

The relatively slight movement of the refining vessel can for instance be a rotation about its longitudinal axis inclined with respect to the horizontal, at a speed equal to or less than 10 rotations per minute.

The said relatively slight movement of the refining vessel can also be brought about by a periodic oscillation about the pivot axis of the said vessel, the said axis being perpendicular to the longitudinal axis of the vessel.

Two operations of obtaining steel with less than 0.015% and 0.025% phosphorus, using the method of the invention will now be described, in no restrictive sense and by way of example only.

EXAMPLE 1 (FOR LOW CARBON STEEL) In a 35 ton converter, 29 tons of pig iron were introduced, having 3.6% carbon and 1.9% phosphorus, together with 7,000 kgs. of scrap. Refining took place in two phases by means of pure oxygen, using the slag remaining from the preceding refining.

(a) First phase 1) First period.-The converter was kept stationary in an upright position for approximately fifteen minutes. During this period there was introduced into the converter 2300 kgs. of lime, of which 1300 kgs., of 6/20 grade, were introduced into the converter directly, whereas 1000 kgs. were introduced in powder form in suspension in the blasting oxygen used for the refining process.

During this period 1600 Nrn. of pure oxygen were blasted into the converter.

At the end of this period the metal contained 0.8% carbon and 0.15% phosphorus.

(2) Second period.-The converter was rotated about its longitudinal axis for approximately 5 minutes, after which intermediate deslagging took place.

During this rotation there was added to the bath 800 kgs. of lime of 2/6 grade, and 300 Nm. of oxygen was blasted-in.

At the intermediate deslagging stage, the temperature of the metal was 1590 C. and the metal contained 0.5% carbon and 0.8% phosphorus, the slag containing 9% iron.

(b) Second phase After the intermediate deslagging there was added to the metal 1200 kgs. of lime of 2/6 grade, 700 kgs. of pellets containing 60% iron, and 500 Nm. of oxygen was blasted-in. This phase lasted 8 minutes and the speed of rotation varied between 10 and rotations per 1 minute. The final temperature of the steel was 1610 C. and it contained 0.03% carbon and 0.013% phosphorus in the converter.

EXAMPLE 2 (FOR HIGH CARBON STEEL) 30 tons of pig iron of the same analysis as in Example 1 were introduced into the converter, together with 8 tons of scrap. Again, the slag remaining from the preceding refining process was used.

(a) First phase (1) First period.The converter was kept upright in a stationary position for approximately 13 minutes. During this period 2300 kgs. of lime were added to the bath, of this, 1500 kgs. of 6/20 grade were poured directly into the converter and 800 kgs. were introduced in suspension in the refining oxygen, and 1170 Nm. oxygen was blasted-in.

(2) Second period.--The converter was rotated about its longitudinal axis for 4 minutes before the intermediate deslagging took place,

While this rotation was going on, 800 kgs. of rock lime were poured into the bath, and 300 Nm. of oxygen was blasted-in.

At the deslagging the metal contained 1% carbon and 0.08% phosphorus, whereas the slag titrated 10% iron.

(b) Second phase After the deslagging, 800 kgs. of lime in rocks were added to the metal, and 550 Nm. oxygen was blasted-in. This second phase lasted for 8 minutes. At the end of the refining process, the steel contained in the converter 0.55% carbon and 0.03% phosphorus, Whereas the slag titrated 20% iron.

In the ingot, the analysis of the steel gave the following result: C, 0.57%; P, 0.022%; Si, 0.25%; S, 0.016%; Mn, 0.75%.

We claim:

1. A method for refining a bath of pig iron contained in a solid-bottomed converter which is rotatable about its longitudinal axis, comprising the steps of (a) blowing an oxidising gas into the converter from above the bath of pig iron while maintaining the converter stationary in an upright position, in a first refining phase,

(b) rotating the converter about its longitudinal axis while continuing the blowing of the oxidising gas in the first refining phase,

(0) removing slag from the converter, and

(d) blowing an oxidising gas into the converter from above the bath of metal, while rotating the converter about its longitudinal axis, in a scond refining phase.

2. A method as claimed in claim 1 in which the blowing of the oxygen containing gas in the first refining phase is commenced with the converter fixed in an upright position and is terminated with the converter rotating about its longitudinal axis, and the blowing of oxygen containing gas in the second refining phase is commenced and terminated with the converter rotating about its longitudinal axis.

3. A method as claimed in claim 1, in which towards the end of the second refining phase, as determined by the carbon content of the metal, a slight additional movement is applied to the converter, to ensure increased contact between slag and metal and to cause the slag to stiffen.

4. A method as claimed in claim 3 in which the said slight movement is rotational movement about the longitudinal axis of the converter which axis is inclined at an angle to the horizontal, at a speed not greater than 10 revolutions per minute.

5. A method as claimed in claim 3, in which the said movement is movement about a pivotal axis which is perpendicular to the longitudinal axis of the converter, the said slight movement being a periodic oscillation about said pivotal axis.

- References Cited UNITED STATES PATENTS 2,853,377 9/1958 Kalling et al --60 3,136,626 6/1964 Decamps 75--52 3,172,756 9/1965 Bengtsson 75--60 FOREIGN PATENTS 583,431 9/1959 Canada. 831,251 3/ 1960 Great Britain.

RICHARD O. DEAN, Primary Examiner.

US. Cl. X.R. 7552 

